1. Field of the Invention
The invention relates to an electric heating device used as an auxiliary heating for motor vehicles that includes a plurality of heating elements, which are combined so as to form a heating block. Each of the heating elements is adapted to be controlled separately to heat a particular portion of a total air flow to be heated. A control device controls the heating power of each of the heating elements separately and is configured such that the allocation of the heating power to each of the heating elements is permuted at predetermined time intervals. Such an electric heating device is particularly suitable for use as an auxiliary electric heating in motor vehicles.
This object is achieved by providing a method of controlling an electric heating device comprising the steps of controlling the heating power of each of the heating elements separately and permuting the allocation of a heating power to each of the heating elements a predetermined time intervals.
2. Description of the Related Art
In motor vehicles, electric heating devices are used for heating the air in the passenger cabin, for preheating the coolant in water-cooled engines or for warming up fuel, among other purposes. Such auxiliary electric heatings normally consist of at least one heating stage with heating elements and a control device. The heating elements are normally implemented as a heating resistor, especially as a PTC element. The heating and the control unit may be implemented as separate functional units, but they may also be combined so as to form one structural unit.
EP-A2 1 157 868 describes an electric heating device in which the heating elements as well as a control unit are combined so as to form one structural unit. For controlling the heating elements, a plurality of control concepts is disclosed, which will be summarized briefly hereinbelow.
A power control for an electric heating device comprises, in the simplest case, a plurality of separate heating elements and an identical control of all heating elements. Such a control is shown in FIG. 1 taking three heating stages as an example. The heating powers of the individual heating stages P1, P2 and P3 are shown one below the other, above the total heating power P (in the lowermost diagram). When the heating demand increases, the individual heating elements will be controlled uniformly so that each of the individual heating elements will produce an increasing heating power. The total heating power P corresponds to the sum of the individual heating powers P1 to P3.
For controlling electric loads, the so-called pulse width modulation (PWM) is frequently used. A characteristic feature of said pulse width modulation is that it can be technically realized in a particularly simple manner. FIG. 2 shows such a clocked control. Each heating circuit of the heating device is clocked by a control unit with a fixed frequency F and the period T. The power of each individual heating element results from the clock ratio. By modulating the width of the pulses, it is possible to vary the heating power.
The power control shown in FIG. 2 corresponds, in principle, to the linear control that has been described making reference to FIG. 1. Hence, all the heating elements are controlled uniformly for producing a predetermined total heating power. When the total heating power increases, the heating power of the individual heating elements will increase accordingly. The clock ratio in FIG. 2 is e.g. 70% for each of the pulses. Hence, 70% of the maximum possible heating power is produced. In the lowermost diagram of FIG. 2, the broken line with the designation P70% indicates the average effective heating power of all heating elements of the heating device, whereas the solid line indicates the respective instantaneous power.
In order to reduce EMC problems in connection with the use of pulse width modulation, the loads are switched on and off “gently”, i.e. with a comparatively slow edge. Since the power switches required for this purpose are, however, controlled in linear operation during such an edge, a substantial instantaneous power loss will be produced simultaneously. Such “edge losses” may amount to an essential percentage of the total power loss at the respective switches in the control of electric auxiliary heatings.
A control of the type shown in FIG. 2 is disadvantageous insofar as the heating power produced by the heating elements varies with time. Another problem are the very high current peaks on the supply line, since all the loads are switched on and off simultaneously.
In order to avoid such variations with time during heat transfer, the heating elements of an electric heating can be controlled with a time shift when pulse width modulation is used. One example for this kind of control is shown in FIG. 3. In this example, the three heating elements shown are clocked with a time shift t. The respective active pulse width is distributed over a whole period T of a clock for the individual stages.
In such a process, the n-fold (n=number of channels) frequency component becomes visible in the sum current of the loads, i.e. of the heating elements. This allows a comparatively low pulse width modulation frequency at a uniform sum current frequency.
When such electric heating devices are used in motor vehicles, the sum current frequency influences the whole onboard power supply of the motor vehicle and can be seen as a disturbing light flicker as soon as the visual perception limits are no longer reached.
As has been mentioned hereinbefore, edge losses will always occur when control is effected via a pulse width modulation. These edge losses occur whenever a load is switched on and off so that their percentage will increase linearly with increasing control frequency. However, the control frequency must not fall below certain lower limits either, so as to prevent the light flicker from becoming visible. Hence, only a certain corridor within which the control frequency can be varied remains for an appropriate control frequency.
The magnitude of the edge losses results from the following equation:                               P          Edge                =                  [                                                    W                                  Rising                  ⁢                                                                           ⁢                  Edge                                                            T                PWM                                      +                                                            W                                      Falling                    ⁢                                                                                   ⁢                    Edge                                                                    T                  PWM                                            ·              n                                ]                                    (        1        )            
In this equation, PEdge stands for the power loss caused by the edges, WRising Edge for the energy converted in a power switch during a rising edge, WFalling Edge for the energy converted in a power switch during a falling edge, TPWM for the period duration of the pulse width modulation and n for the number of channels, i.e. the number of separately controlled heating elements.
Such edge losses can be reduced markedly by improved control methods. In an improved control method for an electric heating device, the heating power of only one of the heating elements is adapted to be variably adjusted for this purpose. All the other heating elements can only be switched on or off, i.e. they can either be operated under full load or under zero load. These heating elements are switched on and off according to requirements. For a “fine adjustment” of the heating power to be generated, the continuously adjustable heating element with a variable heating power contribution is switched on.
When this concept is combined with pulse width modulation, not all the channels are clocked continuously, but only the heating power of the continuously adjustable channel is adjusted through a pulse width modulation. This type of control is shown in FIG. 4 and FIG. 5. The heating power of a heating element is increased until the heating element has reached the maximum of the heating power that can be produced. Subsequently, the current supply to this heating element is continued without clocking, i.e. without pulse width modulation. If the heating power to be produced is increased still further, said heating power will be produced via a pulse width modulation of the next heating element. This process is continued until all heating elements are switched on continuously. FIG. 5 shows an alternative in the case of which only the heating power of one of the heating elements is continuously adjustable, whereas the other heating elements are only switched on and off.
In this way, the same yielded heating power can be produced with lower edge losses. The edge losses occurring are represented by the following equation:                               P          Edge                =                                            W                              Rising                ⁢                                                                   ⁢                Edge                                                    T              PWM                                +                                    W                              Falling                ⁢                                                                   ⁢                Edge                                                    T              PWM                                                          (        2        )            
Due to the fact that only one of the heating elements is controlled via a pulse width modulation at the same time, the edge losses will be reduced to 1/n in comparison with the preceding equation.
A heating power control of the above-mentioned type is, however, disadvantageous with regard to the inhomogeneous heating of the heating block by the individual heating elements. This has the effect that the medium to be heated will be heated in a locally non-uniform manner and will therefore have zones of different temperature.